1. Field of the Invention
This invention is directed to an improved process for preparing coating or adhesive compositions useful in bonding expanded vinyl which is vacuum laminated to molded fiberboard substrates, comprising (1) mixing an aqueous dispersion of an addition polymer containing active hydrogen, wherein the addition polymer contains essentially no polyurethane functional moieties, with water emulsifiable difunctional and/or multi-functional isocyanate and (2) subsequently adding to the mixture an aqueous dispersion of a polyurethane polymer. Adhesive compositions prepared using the process exhibit both superior shear strength and superior hot peel strength at elevated temperatures.
2. Brief Description of the Prior Art
Manufacturers in the transportation industry, especially the automobile industry, build a substantial portion of the interior of their vehicles using parts made from molded fiberboard covered with polyvinyl chloride sheet or film (vinyl). The vinyl film is usually held in place on the fiberboard part with an adhesive. One technique which is commonly employed for covering fiberboard parts with vinyl is vacuum lamination. In the vacuum lamination technique, the vinyl is stretched up to several hundred percent of its original length, being drawn in many cases over very deeply convoluted portions of the fiberboard substrate. Prior to the lamination, the substrate is coated with an adhesive to bond the vinyl to the fiberboard. Because the vinyl is severely stretched during the vacuum lamination process, substantial retractive forces are built into the laminate. The adhesive used to bond the vinyl to the fiberboard must be able to withstand the substantial retractive forces, not only during the manufacturing process, but also after the completed laminated part has been installed in a vehicle. In service, the laminated part may encounter severe environmental conditions repeatedly. For example, an automobile containing laminated parts may well be expected by its owner to withstand sitting in the desert sun with its windows rolled up during mid-summer on a daily basis. The adhesive used to bond the vinyl to the fiberboard must consequently exhibit good resistance to debonding at high temperatures.
Another potential problem which may be encountered in preparing vacuum laminated vinyl/fiberboard parts bonded together with an adhesive is that of plasticizer migration. Polyvinyl chloride films frequently contain substantial proportions of low molecular weight molecules used to plasticize the films. The plasticizer is included in the film in order to make the film flexible. Low molecular weight vinyl plasticizers, for example, the dialkyl phthalate plasticizers, while effective in flexibilizing the otherwise rigid polyvinyl chloride film, suffer from the fact that they are both relatively volatile and tend to migrate from the polyvinyl chloride into materials in which the vinyl is in contact. Over time, plasticizer migrating from the polyvinyl chloride film into the adhesive composition which bonds the vinyl film to a fiberboard substrate may change both the chemical and physical properties of that adhesive composition. The effect may be to weaken the adhesive bond. Plasticizer migration is accelerated at high temperatures, such as encountered in the hypothetical automobile in the desert.
There are two major types of adhesive which are used to bond vinyl to fiberboard in the vacuum lamination process. These same adhesives may be used to bond vinyl to a variety of other substrates, such as wood, metals, other plastics, and other wood fiber products. These adhesives are two-part epoxies and solvent-based two-part urethanes. Although these products give satisfactory performance, there are several problems associated with their use. Among these problems are that of toxicity of the adhesive composition, short pot life (usually on the order of one day), and solvent emission from the adhesives themselves. Although one-part water-based adhesive compositions have long been known for a variety of applications, up until this time water-based adhesives have not been able to provide an adequate combination of heat resistance, plasticizer migration resistance, and humidity resistance necessary for adhesives which are to be used in bonding vinyl to fiberboard substrates in the vacuum lamination process.
Adhesive compositions based on aqueous dispersions of addition polymers prepared by the emulsion polymerization of mono-alpha, beta-ethylenically unsaturated monomers are well known to the art. Homo- and copolymers of acrylic and vinyl acetate monomers are widely used in preparing adhesive compositions. It is known that these polymer latexes may be modified using isocyanate functional monomer or polymers in order to prepare high performance adhesive and coating materials which do not yield formaldehyde upon curing, an undesirable by-product of a widely-used alternative crosslinking chemistry. For example, German Pat. No. 2,643,642, assigned to Koyo Sangyo K.K., discloses an acrylic polymer emulsion containing between 0.1 and 100 weight percent with respect to emulsion polymer solids of isocyanate compound or isocyanate polymer and which is useful as a heat and water resistant pressure sensitive adhesive composition. Another example of isocyanate modification of a polymer emulsion is presented by Japanese unexamined patent application publication No. 79/154,053. This patent discloses a vinyl leather cloth obtained by coating a base cloth (which has been pretreated by an agent composed of an aqueous emulsion and/or the aqueous solution of a water soluble high molecular weight substance and an isocyanate compound or an isocyanate polymer having more than two isocyanate groups per molecule) with a vinyl chloride resin containing plasticizers, stabilizers, pigments, etc. The vinyl leather cloth obtained has excellent softness, water resistance and durability. The preferable isocyanate component is used at 0.1 to 200% by weight on the total solids content of the aqueous emulsion and/or the high molecular weight water-soluble substance. The polymer emulsion may contain polyvinyl chloride vinyl chloride-ethylene copolymer, polyacrylamide, polyacrylate, etc. The water-soluble high molecular weight substance preferably has hydrogen atoms which are reactive with isocyanate; for example, carboxymethyl cellulose, methyl cellulose, polyvinyl alcohol, polyvinyl pyrrolidine, modified polyvinyl alcohol, etc. Another example of isocyanate modified polymer latex is presented by Japanese Kokai No. 75/69137 (Chemical Abstract 83-148465q) which discloses water-resistant adhesive composition for polywoods prepared from an aqueous emulsion mixed with polyvinyl alcohol and an isocyanate compound dissolved in hydrophobic solvent. For example, a butadiene methylmethacrylate copolymer latex can be mixed with polyvinyl alcohol and treated with a mixture of a polyisocyanate, diheptyl phthalate and calcium carbonate filler to give an adhesive which yields plywood with good water resistance and adhesion strength.
Isocyanate functional monomers and isocyanate functional polymers are well known for their reaction with active hydrogen (for example --COOH, --OH, --SH, and NH) in various polymeric and monomeric materials to achieve chain extension, crosslinking and other useful chemical modifications of the primary molecule.
For adhesive applications isocyanate prepolymers have been found to be advantageous in modifying polymer latices. For example Japanese unexamined patent application publication No. 79/102,334 discloses an adhesive composition comprising a water-soluble polymer or aqueous polymer dispersion and a hydrophilic compound containing more than two isocyanate groups per molecule. The adhesive composition shows good water resistance and adhesive strength and can be used for the bonding of plastics. The water-soluble polymer may be carboxy methyl cellulose, hydroxy methyl cellulose, methyl cellulose, polyvinyl pyrrolidone, etc., the aqueous polymer dispersion may be ethylene-vinyl acetate copolymer, polyvinyl acetate, polyacrylic acid, vinyl acetate-acrylic acid copolymer, etc. The hydrophilic compound may be the reaction compound of alkylene oxide polymer.
Another example of this type system is disclosed in Japanese patent application No. 69/11279 (Chemical Abstract 72-13368n) which discloses adhesive compositions having high adhesive strength, aging resistance and chemical resistance, and which is prepared by mixing aqueous emulsions of polymers having carboxyl groups with isocyanate copolymers. For example, an isocyanate block copolymer prepared from a mixture of trimethylolpropane toluene diisocyanate and MeEtC.dbd.NOH can be emulsified and added to a latex polymer prepared from methacrylic acid, methyl methacrylate, ethylene glycol monomethacrylate and butadiene, to give an adhesive composition useful in coating fabric.
Another example is disclosed by Japanese examined Pat. No. 76/036,264 in which a vinyl acetate and/or (meth)acrylate ester (co)polymer or its mixture with polyvinyl alcohol is emulsified and an isocyanate prepolymer is added to the macromolecular emulsion to produce a coating composition. The prepolymer is prepared from polyoxypropylene glycol or polyoxypropylene triol and toluene diisocyanate.
U.S. Pat. No. 4,396,738 discloses aqueous adhesive and coating compositions composed of an aqueous emulsion or dispersion of a polymer or interpolymer of one or more vinyl monomers (such as an emulsion of a copolymer of vinyl acetate and butyl acrylate) and a aqueous emulsion or dispersion of polyisocyanate which has at least partially reacted with an alcohol, and wherein the dispersion or emulsion of the polyisocyanate is stabilized with a combined surfactant and alcohol.
Enhanced performance of isocyanate modified polymer latex is generally considered to be derived from chemical crosslinking.
Aqueous dispersions of polyurethane resins are well known in the art. For example, a preparation of polyurethane dispersions is disclosed in U.S. Pat. No. 4,186,118, which is hereby incorporated by reference, and in German Offenlegenschriften Nos. 2014385, 1953345, 1953348, and 1953349. Polyurethane resin dispersions are known to be useful in combination with resin emulsions of other types of copolymer. For example, Japanese unexamined Pat No. 79/127,442 (Derwent Abstract No. 81792B/45) discloses adhesive compositions comprising an aqueous solution or dispersion of a polyester polyol-type polyurethane and a carboxy-modified rubber latex or copolymer emulsion which shows improved heat resistance and reduced tendency to lose adhesion to a substrate even on heating at high temperature over a long period of time. A polyurethane solution or dispersion is used in the amount of 5-100 parts by weight on the solids content relative to a 100 parts by weight of a carboxyl modified synthetic rubber latex or ethylene vinyl acetate copolymer emulsion. The polyurethane may be, for example, prepared from MDI and polyester polyol which in turn may be prepared from isophthalatic acid and tetramethylene glycol or adipic acid and 2,2-dimethylolpropionic acid. Another example is given by U.S. Pat. No. 4,108,811, which is hereby incorporated by reference. The U.S. Pat. No. 4,108,811 discloses a water-based latex composition comprising a binder formed by blending an aqueous disperion of an acrylic or vinyl acetate polymer and an aqueous alkaline emulsion of a polyurethane obtained by reacting diisocyanate with a hydroxylic oil-modified resin (which is either the alcoholysis product of a drying or semi-drying oil) and at least one polyol or the reaction product of at least one polyol or at least one mono- or polymeric drying or semi-drying fatty acid. This composition is useful for coating corrodable surfaces, for example, sand-blasted ferrous metals and is stable even in the presence of highly reactive pigments.
U.S. Pat. No. 4,186,118 discloses a process wherein a diisocyanate is used to modify an aqueous mixture of polyurethane dispersion and a non-polyurethane polymer latex. The modification is accomplished by coating the non-polyurethane polymer latex particles with a polyurea film formed from the diisocyanate. These modified dispersions are useful as water resistant surface coatings and as leather dressings which are abrasion resistant, even in the presence of moisture.
U.S. Pat. No. 4,347,338 discloses a process for preparing a thermosettable polyurethane product which comprises blending a compound having two or more terminal isocyanate groups (e.g. 4,4'-diphenylmethane diisocyanate) with a thermoplastic resin (e.g. acrylic resin) which is inert towards the isocyanate groups to prepare an isocyanate compound batch and further blending the isocyanate compound batch with a thermoplastic urethane resin, and subsequently fabricating the resultant mixture, as by melt extrusion. The thermosettable product may be formed into a variety of useful articles and cured by heat application.
U.S. Pat. No. 4,433,095 discloses improved aqueous adhesives comprising an aqueous polymer dispersion and a water-dispersible polyisocyanate having an average NCO-functionality of at least 2.2 and comprising an aromatic isocyanate and an emulsifier produced by reacting an aromatic isocyanate with hydrophilic compounds such as polyether alcohols.
U.S. Pat. No. 3,931,088 discloses an adhesive composition comprising (1) aqueous solution of polyvinyl alcohol, aqueous emulsion of vinyl acetate polymer and/or an aqueous latex of butadiene polymer; and (2) solution of an isocyanate compound or polymer. Further U.S. Pat. No. 3,401,135 discloses adhesive composition consisting of (1) latex containing copolymers of polymerizable olefinic compound, acrylic or methacrylic acid, and ethylene glycol monomethacrylate; and (2) a blocked polyisocyanate. Both the U.S. Pat. Nos. 3,931,088 and 3,401,135 only teach adhesive compositions comprising a polymeric latex and an isocyanate compound, and do not incorporate the beneficial polyurethanes in the formulations.
U.S. Pat. No. 4,298,511 describes a urethane rheology modifier derived from the reaction of polyakylene oxide; polyfunctional compounds, such as polyols, amines, and thiols; diisocyanate; and water. The reaction takes place in an inert organic solvent where all reactants are blended together or in any order. These modifiers can be used in water- or solvent-based coating compositions, such as acrylic latexes.
Aqueous polyurethane dispersions (PUD) of linear polyester urethanes are known to be useful as laminating adhesives, especially where flexibility, light resistance and adhesive strength at high temperatures are desired. PUDs can be used to bond textiles, non-wovens, leather, fiberboard and polyurethane films as well as plasticized polyvinyl chloride films. PUD can be blended with other polymer dispersions such as latices of acrylic monomers, ethylene-vinyl acetate, stryrene-butadiene rubbers, nitrile and natural rubbers, and the like. Blends may be chosen to enhance the flexibility of a PUD film or improve its toughness. PUD may be plasticized with conventional plasticizers such as the phthalate-ester plasticizers and may be crosslinked, as with urea and melamine/formaldehyde condensates.
In the present invention the applicant has surprisingly discovered that improvements in polyurethane-modified isocyanate/polymer coatings and adhesives can be obtained through a specific sequence of addition/reaction. Applicant's invention involves a two-step process wherein (1) latex polymer containing active hydrogen is mixed with multi-functional isocyanates to form a polymer-isocyanate adduct, and (2) the adduct is subsequently mixed with the polyurethane to form the final polymer-isocyanate-polyurethane composition. This two-step sequence results in adhesive and coating compositions having surprisingly improved adhesion properties.